Smooth-On Mold Star 15 Slow - 1A:1B Mix by Volume Platinum Silicone Rubber - Pint Unit

I've tried a lot of molding silicones from a few brands, and so far, this one's my favorite for a handful of reasons. 1. Long pot life means there's ample time to mix and pour before things start to thicken up on you. If you're like me, when you first start making your own molds, you'll find dozens of ways to screw things up. A nice long pot life means that if (or in my case, WHEN) things go haywire, you'll have plenty of time to fix things. Silicone isn't cheap, so mistakes can be expensive -- the wiggle room is nice! 2. Relatively quick cure time, especially given how long you have before the mix starts to solidify. I can come home from work, pour a mold, and it's ready to use with time to spare before I need to head to bed. 3. Platinum cure is always a plus. For mysterious reasons, I've had trouble with tin cure silicones and the material I usually cast (epoxy). I think post-curing tin-cure molds is supposed to solve part of the issue, but even when I've tried that, the tin-cure molds have left a slightly tacky, sticky film on the surface of my castings. I've had much better luck with platinum silicones, which actually seem to *improve* how my epoxy resin cures. 4. Low viscosity means you don't get bubbles as long as you don't overmix, and the trapped air in detailed molds doesn't have shove its way through super-thick goop to break free. In other words, unlike a lot of silicones, you can get excellent results with this one without using a vacuum degassing thing-a-ma-bob. The last thing is a plus/minus: This stuff is soft, but not insanely soft. The flexibility makes it excellent for intricate stuff because you don't need Hercules hands to get your casting out of the mold. However, really soft, flexible silicone like this can also cause some problems. So here are a couple tips I learned the hard way :). 1. If you'll be casting something in multiple layers, make sure you either place your mold in some kind of support shell -- a cup or a plastic bowl can work -- or make your mold with extra "buffer" rubber on the base and the sides. Why? Well, if you move the mold too much, those awesome natural silicone release properties can work against you and knock your layer loose, which means the next layer can ooze down the sides. Having something to make sure the soft mold doesn't get jostled makes all the difference. 2. On a similar note, if you're going to make one mold with lots of cavities, it's probably best to time things to everything is ready to be popped out of the mold at the same time. The flexibility means it's pretty easy to accidentally knock one casting loose while trying to remove another one. All in all, a really awesome product. Highly recommended!

I used this to mold ornate chess pieces and to make some cuppboard knobs in PU resin. It mixes simply and don't have to hurry (in fact I might look for a slightly faster version). I cast using a plastic cup and a paper cyclinder as a container and both worked fine and could be removed. neither of the masters (the resin chess pieces of wooden knobs) got stuck at all. The resulting mold has good, but flexibility but is fairly dense, the knob needed a split mold to get the bulbous part out. I cast with PU resin without any mold release spray and they came out of the mold easily and the level of detail was exceptional.

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I love this product, it’s brilliaht and easy to use. Although very messy, make sure you wear gloves. Just wish it wasn’t to expensive.

Mold Star 15 Slow has been fantastic for my projects. It captures fine detail really well and gives me consistent, bubble-free results. The longer working time is a huge plus when I need extra time to get everything just right. But what really stood out was the customer support. I reached out to Smooth-On directly to figure out which silicone would be best for what I was doing, and they walked me through everything step by step. Later, when I had some trouble with my mixing technique, they were super helpful and patient—totally saved me from wasting materials. Great product backed by a great team. Highly recommend it if you're getting into mold making or want something reliable with solid support behind it.